Tagged Questions
3
votes
3answers
46 views
Showing Hubble constant is time-independent
I have the following question for homework:
Show that the Hubble constant $H$ is time-independent in a universe in which the only contribution to energy density comes from vacuum energy.
So in ...
1
vote
1answer
65 views
Assuming space is infinite can our observable universe be an island amongst an archipelego?
According to recent measurements our observable universe is roughly 93 billion light years in diameter; also it appears (according to WMAP measurements) that spacetime is flat.
Supposing space is ...
1
vote
2answers
166 views
Excluding big bang itself, does spacetime have a boundary?
My understanding of big bang cosmology and General Relativity is that both matter and spacetime emerged together (I'm not considering time zero where there was a singularity).
Does this mean that ...
3
votes
2answers
98 views
Is time going backwards beyond the event horizon of a black hole?
For an outside observer the time seems to stop at the event horizon. My intuition suggests, that if it stops there, then it must go backwards inside.
Is this the case?
This question is a followup ...
0
votes
1answer
90 views
What is the curvature of an empty universe?
My calculations tell me an empty universe has hyperbolic curvature. Is this correct? If it is, can anyone help me understand why this is intuitively?
1
vote
1answer
83 views
Ricci scalars for space and spacetime, local and global curvature
If Ricci scalar describes the full spacetime curvature, then what do we mean by $k=0,+1,-1$ being flat, positive and negative curved space?
Is $k$ special version of a constant "3d-Ricci" scalar?
...
4
votes
2answers
111 views
Can dark matter be relativistic dust?
As far as I know the mass of an observed object increases as it approaches the speed of light.
Is it possible that the excess mass called "dark matter" is due to relativistic dust?
Surely, stars ...
4
votes
1answer
96 views
Why don't orbits expand with the Universe?
Consider two bodies orbiting each other.
As the Universe expands would the distance between them increase?
Most people say that a gravitationally bound system will not expand with the Universe. They ...
1
vote
2answers
78 views
Coordinate and conformal transformations of the FRW metric
I'm considering a metric of the following form (signature $(+,-,-,-)$):
$$ds^2 = (F(r,t)-G(r,t))dt^2 - (F(r,t)+G(r,t))dr^2 - r^2(d\Omega)^2$$
where $F(r,t)$ and $G(r,t)$ are arbitrary scalar ...
1
vote
1answer
70 views
Can the fuzzball conjecture be applied to microscopically explain the entropy of a region beyond the gravitational observer horizon?
In this article discussing this and related papers, it is explained among other things, how the neighborhood of an observer's worldline can be approximated by a region of Minkowsky spacetime.
If I ...
2
votes
1answer
176 views
Cosmological constant
I have always wondered about how cosmological constant is characterized. So since it is still a hypothesis you often read the “cosmological constant measured to be ….”. Shouldn't the statement read ...
0
votes
2answers
134 views
Advanced Heaviside-Feynman formula implies electromagnetic inertia?
The Heaviside-Feynman formula (see Feynman Lectures vol I Ch.28, vol II Ch. 21) gives the electric and magnetic fields measured at an observation point $P$ due to an arbitrarily moving charge $q$
$$ ...
1
vote
0answers
51 views
Second order action ADM formalism
I am trying to derive the second order action
$$S_{(2)}~=~\frac{m_{pl}^{2}}{8}\int a^{2}[(h_{ij}')^{2}-(\partial_{i}h_{ij})^{2}]d^{4}x, $$
used for tensor fluctuations derived from the ADM ...
1
vote
2answers
91 views
Does the distance to the cosmic horizon Lorentz-contract? Does the universe Lorentz-contract?
Our universe has a finite size. It is often called the "radius of the universe", or "distance of the cosmic horizon".
If we would fly with relativistic speed at the position of our Earth, would this ...
5
votes
0answers
87 views
Equation of state of cosmic strings and branes
I'm sure these are basic ideas covered in string cosmology or advanced GR, but I've done very little string theory, so I hope you will forgive some elementary questions. I'm just trying to fit some ...
1
vote
2answers
77 views
Hamiltonian constraint in spherical Friedmann cosmology
I'm taking a GR course, in which the instructor discussed the 'Hamiltonian constraint' of spherical Friedmann cosmology action. I'm not quite clear about the definition of 'Hamiltonian constraint' ...
1
vote
0answers
41 views
The definition of $f_{NL}$ and transfer function
To me there seems to be quite a few different definitions of $f_{NL}$ in cosmology and I would like to know if or how they are equivalent. Let me cite at least 3 such,
One can see the equation 6.71 ...
1
vote
1answer
201 views
Dirac Equation in General Relativity
Dirac equation for the massless fermions in curved spase time is $γ^ae^μ_aD_μΨ=0$, where $e^μ_a$ are the tetrads. I have to show that Dirac spinors obey the following equation:
...
3
votes
3answers
220 views
Question on inflation
I have two particular questions regarding the inflationary scenario. They are:
1.) What is the physical origin of the inflaton field?
2.) Why has the potential of the inflation field its particular ...
10
votes
5answers
482 views
Does the amount of gravitational potential energy in the universe increase as it expands?
It seems to me that extra gravitational potential energy is created as the universe expands and the distance between massive objects such as galaxy clusters increases; this implies that energy is not ...
3
votes
1answer
125 views
Question on inflation as a phase transition
I have just finished watching the following video http://www.youtube.com/watch?v=beQ9fZ0jVdE where Laughlin, Gross and some students discuss e.g. about inflation. The following question is risen:
Is ...
1
vote
0answers
183 views
Curiosity episode with Stephen Hawking. The Big-Bang
In an episode of Discovery's Curiosity with host Stephen Hawking, he claims the Big Bang event can be explained from physics alone, and does not require the intervention of a creator.
1) His ...
3
votes
0answers
75 views
Conservation of Energy in the Universe [duplicate]
Possible Duplicate:
Is energy really conserved?
Why can’t energy be created or destroyed?
One of the laws of the universe that dazzles me the most is the law of conservation of energy. I ...
1
vote
2answers
217 views
Cosmology questions from a novice
These ideas/questions probably represent a lack of understanding on my part,
but here they are:
1) Cosmologists talk about the increasing speed of expansion of the universe and talk of dark energy as ...
1
vote
3answers
206 views
Mechanism for the gravitational field generated by photons
This question follows from a schooling I received in this thread.
I figured that photons do not interact with gravity, except when they've spontaneously converted into a particle-antiparticle pair. ...
6
votes
3answers
447 views
Is the total energy of the universe constant?
If total energy is conserved just transformed and never newly created, is there a sum of all energies that is constant? Why is it probably not that easy?
5
votes
2answers
198 views
Einstein tensor in Friedmann equations : where is the missing $c^2$?
I would like to demonstrate the several forms of the Friedmann equations WITH the $c^2$ factors. Everything is fine ... apart that I have a missing $c^2$ factor somewhere.
In all the following $\rho$ ...
2
votes
1answer
126 views
Cosmology with a negative cosmological constant
Based on the Friedmann equation for a universe with only cosmological constant,
$$\left(\frac{\dot{a}}{a}\right)^2 \sim \Lambda$$
I would expect the scale factor $a(t) \sim e^{-it}$ if $\Lambda < ...
3
votes
1answer
141 views
Why are black hole singularities stable?
The Friedmann equations says that huge matter densities lead to huge expansion rates. In Newtonian gravity, two massive point particles separated by an infinitesimal distance will experience an ...
0
votes
1answer
73 views
What is meant by the term “mean rest frame”?
I found this part in the book of Bernard Schutz on GR. (Sec 12.2: General Relativistic Cosmological Models).
Robertson-Walker metrics: We shall adopt the following assumptions about the universe: ...
8
votes
0answers
290 views
Do intergalactic magnetic fields imply an Open Universe?
According to a recent paper on the arXiv, they do. How credible is this result? The abstract says:
The detection of magnetic fields at high redshifts, and in empty
intergalactic space, support ...
3
votes
1answer
104 views
How do black holes in merging galaxies find each other to merge?
In many accounts of galaxy mergers, the prompt merging of their central black holes, if any, is stated seemingly as too obvious to need further explanation.
While I don't dispute that this may indeed ...
4
votes
0answers
130 views
Implications of Unruh-inertia to theories of gravity
If it turns out to be true that the galaxy rotation curves can be explained away by Unruh modes that become greater than the Hubble scale at accelerations around $10^{-10} m/s^2$ as proposed in here, ...
3
votes
2answers
97 views
Cosmological relativistic effects : misunderstanding between cosmological and relativistic communities?
I would like to clarify something that mixes cosmology and relativistic effects. Maybe I'm not understanding something or maybe there a difference of vocabulary between the cosmological and the ...
1
vote
1answer
171 views
Do atoms expand with universe? [duplicate]
Possible Duplicate:
Why space expansion affects matter?
Why does space expansion not expand matter?
As we know, the universe is expanding, galaxies are away from each other. But what ...
1
vote
5answers
810 views
Is the law of conservation of energy still valid?
Is the law of conservation of energy still valid or have there been experiments showing that energy could be created or lost?
2
votes
1answer
102 views
variations of Einstein equations with conversion between gravitational and non-gravitational energy
I'm looking for existing papers studying a variation to Einstein equation that does not rely on the annoying matter conservation identity:
$$ T_{\mu \nu; \nu} = 0 $$
And instead tries to equate the ...
-1
votes
1answer
150 views
What if microstates increase proportional to universe volume?
I am probably a delusional crank with a lot of crazy, overly speculative conjectures. If I am not delusional, than at the very least I've been ahead of the curve, the last 40 or so years. I was a ...
0
votes
1answer
52 views
Can anything come out from the big bang?
If any configuration of matter can fall into a black hole and hit the singularity, and ditto for the big crunch, and there is time reversal CPT invariance, does it mean anything can pop out of the ...
2
votes
4answers
188 views
Space Expansion vs. Relative Motion
Inspired by a recent dialogue from another question:
Given 2 objects moving at some velocity $v$ relative to one another, is it possible to determine whether they are moving or whether the space ...
2
votes
0answers
67 views
Is eternal inflation Lorentz invariant?
Start without general relativity. Consider a metastable vacuum over good ol'-fashioned Minkowski space. It decays. A bubble forms and the domain wall expands. The domain wall is timelike, and ...
5
votes
1answer
144 views
variable speed of light in cosmology
In this paper, D. H. Coule argues that warp drive metrics, like the one proposed by Alcubierre, require the exotic matter to be laid beforehand on the travel path by conventional travel. At section 5 ...
1
vote
0answers
61 views
Reference request: FLRW with k>0, dust, and positive cosmological constant
The exact solution representing a FLRW universe with $k>0$ and dust (p=0), and $\Lambda=0$, is described by a cycloid. What is the exact solution for dust, in the presence of a positive ...
3
votes
1answer
122 views
How can a deSitter space have finite size?
a deSitter space is a maximally symmetric solution of Einstein equations, I have some problem picturing one thing: this space is past and future (time) infinite but spatial slices have finite size, ...
3
votes
3answers
157 views
What does Brian Greene mean when he claims we wont be able to observe light from distant stars due to the universe's expansion?
Brian Greene in this TED talk about possible multiverse, claims tomwards the end (At around 18:00 mark) this statement. 'Because the expansion is speeding up, in the very far future, those galaxies ...
1
vote
0answers
115 views
How is the poincare conjecture(and perelman proof) helpful in studying the properties of the universe?
Can someone tell me how the poincare's famous conjecture or its proof by perelmen can be helpful in deciding some properties like the shape of the universe?
2
votes
1answer
96 views
extracting energy from cosmological expansion
This question is a more concrete reincarnation of an old question about energy conservation in GR.
Are there mechanisms to extract energy from the cosmic rate of expansion? putting some extremely ...
3
votes
2answers
416 views
Zero divergence of energy-momentum tensor and gravitational energy
Trying to teach myself general relativity and have just hit yet another confusion. I'm reading that in curved spacetime the energy-momentum tensor has zero divergence, ie
...
1
vote
2answers
209 views
How is Big Bang related to theory of relativity?
I'm not someone with good scientific knowledge, so if my question are weird, correct me.
I was reading about big bang and I came by the theory of relativity. Can someone explain the relation between ...
1
vote
1answer
210 views
spacetime expansion and universe expansion?
First of all, does the expansion of spacetime solely cause the expansion of universe?
Secondly, if spacetime is the sole cause, do objects(matter with mass) themselves expand?
Thirdly, by spacetime ...
